1. Field of the Invention
This invention relates to loudspeaker drivers and, more particularly, to compression drivers for horn speakers.
2. Description of the Prior Art
Drivers which convert an electrical signal into acoustical energy or sound waves and which radiate the sound waves into the air are well known. Such devices are generally broken down into direct radiators, which directly radiate the generated sound waves, and indirect radiators, which require additional elements for radiating the generated sound waves. In a typical arrangement, the conductor for the electrical signal is a coil of thin wire mechanically connected to a flexible diaphragm and positioned in a magnetic field. As an electrical current passes through the coil, mechanical forces are exerted on the coil and cause the diaphragm to move or vibrate and generate the sound waves. In a direct radiator, the diaphragm directly vibrates or moves the surrounding air and generates the sound waves related to the electrical signal. In an indirect radiator, the diaphragm moves against a surface closely spaced thereto and generates high pressure compression waves which are passed through a throat and to a horn or other acoustic generator having a smaller upstream area than the diaphragm. Generally, indirect radiators, such as compression drivers, can generate much higher audible levels when compared with direct radiators and are used, for example, in public address systems.
A typical compression driver for a horn includes a dome-shaped diaphragm positioned above a bore or throat through a pole piece made of a ferromagnetic material. Positioned between the diaphragm and the upstream end of the pole piece throat is a phase plug having an outer surface closely spaced from and conforming identically to the shape of the diaphragm. The phase plug is used to reduce the volume of air to be compressed by the vibrating diaphragm and, thus, reduce the overall impedance of the driver. The phase plug is also used to eliminate certain interfering cancellations in the generated sound waves through carefully designed and positioned passageways or holes through the phase plug. Various passages have been provided in phase plugs, including stacked cone-shaped plugs having annular passages therebetween, arrays of holes and radial slots. For representative patents on compression drivers, see U.S. Pat. Nos. 3,432,002; 4,152,552; 4,336,425; 4,348,549; 4,531,608; 4,718,517; 4,836,327; 4,975,965 and 5,117,462. See also, "Handbook For Sound Engineers", The New Audio Cyclopedia, First Edition, 1988, pages 445-455.
While a phase plug is generally essential to the efficient operation of a compression driver, a phase plug is the direct cause of several major problems in compression drivers. Since a long path exists from the outer periphery of the diaphragm to the horn annulus, by way of the phase plug, the generated sound wave is distorted and phase problems and cancellations are created. Moreover, the phase plug adds undesirable second and higher order distortion as well as undesirable non-linear distortion. In addition, since the phase plug must be located extremely close to the diaphragm, excursions of the diaphragm are limited and low frequency generation is compromised. Finally, the upper frequency range of typical compression devices is limited to about 18 kHz. While humans generally cannot hear frequencies at or above this level, the inclusion of the higher frequencies makes the overall sound signal sound more lifelike, particularly in public address systems.
It is, therefore, an object of the present invention to provide a compression operated horn driver which overcomes these drawbacks of the prior art devices. It is an object of the present invention to provide a horn driver which has an extremely wide bandwidth, with increases at both the lower and higher frequencies, which has excellent phase coherence, which has extremely low distortion, and which has a significantly lower nonlinear distortion. It is an object to provide such a horn driver in an arrangement which has less mass, which equalizes the path to the horn annulus, in which the diaphragm assists in dissipating heat, which allows for higher power handling capability and less power compression, and which does not require the diaphragm to be made of an exotic or expensive material.